1. Field of the Invention
This invention relates to a semiconductor laser device having a diffraction grating, which attains laser oscillation in a single longitudinal mode at a wavelength of 580 to 890 nm, such as a distributed feedback semiconductor laser device and distributed Bragg reflection semiconductor laser device.
2. Description of the prior art
In cases where semiconductor laser devices are used as a light source in an optical information transmission system, optical measurement system, or any other system utilizing optical fibers, these semiconductor laser devices are preferred to have the operating characteristics of oscillating in a single longitudinal mode. Examples of the well-known semiconductor laser devices which attain laser oscillation in a single longitudinal mode are distributed feedback semiconductor laser devices and distributed Bragg reflection semiconductor laser devices, in which a diffraction grating with a periodically corrugated pattern is formed over the active region or in the vicinity of the active region, respectively.
FIG. 2 shows a conventional distributed feedback semiconductor laser device, wherein on the plane of an n-InP substrate 11, an n-InP first cladding layer (i.e., buffer layer) 12, an undoped InGaPAs active layer 13, a p-InGaPAs optical guiding layer 14, a p-InP second cladding layer 15, and a p-InGaPAs cap layer 16 are successively grown. On the upper face of the cap layer 16 and the back face of the substrate 11, p-sided and n-sided ohmic electrodes 17 and 18 are formed, respectively. The optical guiding layer 14 has, on its surface, a diffraction grating for laser oscillation. Such an InGaPAs/InP laser device which can attain laser oscillation at a longer wavelength of 1,300 nm among semiconductor laser devices has been used in optical fiber communication.
In contrast, distributed feedback semiconductor laser devices and distributed Bragg reflection semiconductor laser devices with a similar structure, which have an oscillation wavelength of 890 nm or less, can also be considered. These semiconductor laser devices may have a structure in which on the plane of an n-GaAs substrate, an n-Al.sub.x Ga.sub.1-x As first cladding layer, an undoped Al.sub.x Ga.sub.1-x As active layer, a p-Al.sub.x Ga.sub.1-x As optical guiding layer, a p-Al.sub.x Ga.sub.1-x As second cladding layer, and a p-GaAs cap layer are successively grown. In such a structure, the p-Al.sub.x Ga.sub.1-x As second cladding layer is to be grown on the diffraction grating which has been formed on the surface of the p-Al.sub.x Ga.sub.1-x As optical guiding layer. Therefore, it has been believed that the growth of semiconductor crystals on the Al.sub.x Ga.sub.1-x As layer is difficult, because semiconductor crystals containing aluminum (Al), such as Al.sub.x Ga.sub.1-x As, are susceptible to oxidation in air to form an oxide film on its surface. However, the inventors have found that the excellent crystal growth on the Al.sub.x Ga.sub.1-x As layer (the Al mole fraction x meeting the relation 0.ltoreq.x.ltoreq.0.4) having a diffraction grating can be achieved by liquid phase epitaxy without any oxygen leakage. With the use of this technique, the inventors have realized a distributed feedback Al.sub.x Ga.sub.1-x xAs laser device with an oscillation wavelength of 780 nm.
However, even when a diffraction grating made of Al.sub.x Ga.sub.1-x As (x=0.3) is used, which has the greatest band gap in the Al.sub.x Ga.sub.1-x As crystals capable of yielding excellent crystal growth, the laser oscillation cannot be obtained at a wavelength of less than 740 nm, because the band gap of the active layer becomes smaller than that of the optical guiding layer having the diffraction grating. Therefore, the technique for producing a semiconductor laser device having a diffraction grating, which attains laser oscillation at a wavelength of 740 nm or less, has not yet been established. On the other hand, in the case of distributed feedback semiconductor laser devices with an oscillation wavelength of 740 nm or more wherein an Al.sub.x Ga.sub.1-x As optical guiding layer having a diffraction grating is disposed in the vicinity of the active layer, these semiconductor laser devices have poor temperature characteristics, because the optical guiding layer has a small Al mole fraction x and therefore has a small band gap, so that satisfactory carrier confining effects cannot be obtained.